214 STUDIES IN GENERAL PHYSIOLOGY 
space ts also of importance for its organization. For it 
is possible to control organization in Tubularia by contact 
stimuli to such an extent that by their help we can pre- 
scribe in a large measure whether a root or a polyp shall 
grow from the aboral cut end of a Tubularian. If the 
aboral end of a piece of Tubularian stem is surrounded by 
water, so that it does not rub against the surface of 
solid bodies, almost without exception a polyp is formed 
upon it instead of a root; heteromorphosis occurs instead 
of regeneration. If, however, the same end is brought into 
permanent contact with the surface of a solid body—if, 
for example, the stem is laid horizontally upon the bottom 
of the vessel—a root is often formed. But to accomplish this 
it is necessary to keep the dish quiet, and care must be taken 
that the same side of the stem remains in contact with the 
solid body; otherwise it does not attach itself. If the stem 
does not attach itself, a polyp is soon formed, just as a polyp 
is formed at the tip of a Tubularian root which has been 
freed from the bottom of the vessel, as soon as it is sur- 
rounded by water. Of course, it is in every case necessary 
that the general conditions of growth of the animal, which 
we shall discuss later, be fulfilled. I remember that only 
once in hundreds of individual experiments did I see a root 
grow without contact with a solid body.’ Buta factor came 
into play here which still brings this case under the 
general grouping. I had cut a section from the stem of 
a Tubularian, and had laid it horizontally upon the bot- 
tom of a glass vessel. A root sprang from the aboral end 
which attached itself to the bottom of the vessel. Simul- 
taneously with this root arose a second, which curved up- 
ward somewhat, and which continued to grow asa root with- 
out contact with a solid body. The exception was probably 
1In recent experiments on Tubularia crocea I have frequently observed the for- 
mation of roots at the aboral end, even if the latter was not in contact with a solid 
body. [1903] 
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